1. Field of the Invention
The present invention relates generally to an improvement of a ladder-type piezoelectric filter constructed using a plurality of series resonators and parallel resonators, and more particularly, to a ladder-type piezoelectric filter in which the directions of polarization of resonators respectively constructed using piezoelectric plates are devised.
2. Description of the Prior Art
FIG. 2 shows one example of the known ladder-type piezoelectric filter. In this ladder-type filter, four series resonators 1 to 4 are connected in series between input and output terminals (IN and OUT) to thereby constitute a series arm. On the other hand, parallel resonators 5 to 7 are respectively connected between nodes of the series resonators 1 to 4 and a reference potential (G) to thereby constitute three parallel arms.
As respective resonators constituting the series resonators 1 to 4 and the parallel resonators 5 to 7, a resonator has been generally used in which a piezoelectric plate, for example, a piezoelectric ceramic plate is subjected to polarization processing in the direction of thickness and electrodes are formed on both major surfaces of the piezoelectric plate. Although in a circuit configuration shown in FIG. 2, four series resonators 1 to 14 and three parallel resonators 5 to 7 are used, the number of resonators and the number of stages of the filter are suitably altered in accordance with objective filter characteristics.
Meanwhile, in the ladder-type filter, the directions of polarization of piezoelectric plates of the respective resonators constituting the series resonators 1 to 4 and the parallel resonators 5 to 7 have not been conventionally considered. Japanese Patent Laid-Open Gazette No. 212212/1988 only discloses a ladder-type filter of such construction that the directions of polarization of piezoelectric plates of series resonators are made constant.
As a result, a product is sometimes made in which the directions of polarization P.sub.1 to P.sub.4 of the piezoelectric plates of the series resonators 1 to 4 are the same, while the directions of polarization P.sub.5 to P.sub.7 of the parallel resonators 5 to 7 are not the same, as represented by arrows P.sub.1 to P.sub.7 in FIG. 2. If the temperature of the piezoelectric plates is changed due to the change of the ambient temperature, high voltages are induced due to the pyroelectric effect of the piezoelectric plates. Consequently, when the directions of polarization P.sub.1 to P.sub.7 of the respective resonators are as shown in FIG. 2, the high voltages induced due to the pyroelectric effect of the piezoelectric plates respectively used for the resonators 1 to 7 are liable to be synergistically applied between the input and output terminals and the reference potential.
As a result, when a rapid change in temperature is made in the ladder-type filter at the time of soldering or the like, the high voltages induced due to the pyroelectric effect are liable to adversely affect an active device connected to the input or output terminal. Particular if a withstand voltage of the active device connected is low, the active device may, in some cases, be destroyed.